主句与从句的动作发生在不同的时间，这时主，从句谓语动词的虚拟语气形式因时间不同而不同，这叫做混合条件句。
If you had asked him yesterday, you would know what to do now.
　 （从句与过去事实相反，主句与现在事实相反。）
If it had rained last night (过去), it would be very cold today (现在).